1. Field of the Invention
The present invention relates to a photomask, which is employed for working a workpiece such as a semiconductor wafer through a lithographic technique.
2. Description of the Prior Art
FIG. 6 is a schematic block diagram of an exposure apparatus including a photomask which is generally used in a photolithographic process, and FIG. 7 is an enlarged view of its essential part.
As shown in these figures, light L.sub.1 outgoing from a light source 1 is focused by a lens system 2 and emitted onto a photomask 3. In the photomask 3, a light shielding pattern 5 is formed on one major surface of a transparent substrate 4, so that light entering a region corresponding to the light shielding pattern 5 is cut off within light L.sub.2 which is incident upon the photomask 3, while light entering the remaining region is transmitted. Light L.sub.3 selectively transmitted through the photomask 3 is collected in a resist film 8 which is formed on a substrate 7 serving as a workpiece, through a projecting lens system 6 of a magnification m which is finished telecentrically, for example. Thus, the resist film 8 is partially photosensitized so that the mask pattern is transferred onto the resist film 8.
In the conventional photomask 3, however, the surface thereof is substantially formed into a plane, so that each light L.sub.3 selectively transmitted through the photomask 3 forms an image on a focal plane 9 through the lens system 6. On the other hand, the surface of the substrate 7 serving as the workpiece is formed into an irregularity corresponding to an integrated circuit pattern formed on the substrate 7, so that the resist film 8 is also formed into an irregularity corresponding to the surface of the substrate 7. For example, when a convex region 7a shown in FIG. 7 is formed on the surface of the substrate 7, it produces a difference in level between the regions 8a and 8b of the resist film 8. In this case, when the focal point of the lens system 6 is adjusted on one region 8a, the other region 8b is out of the focal point thereof, while, when the focal point thereof is adjusted on the other region 8b, one region 8a is out of the focal point thereof, so that a defectively photosensitized region is inevitably caused in the resist film 8.
Continuing with the explanation more precisely, a prescribed depth of focus DOF expressed in the following equation is recognized in the lens system 6 with respect to light of a wavelength .lambda., assuming that NA represents its numerical aperture: ##EQU1##
Since the resist film 8 is substantially photosensitized within the range of the aforementioned depth of focus DOF about the focal plane 9 along the thickness direction of the resist film 8, the aforementioned problem will not be caused if the value of the depth of focus DOF is sufficiently large with respect to the irregularity of the resist film 8.
With recent refinement of LSI, however, the numerical aperture NA tends to increase in consideration of the converging ability of the lens system 6. Consequently, the depth of focus DOF tends to decrease to the contrary. In a current manufacturing process for LSI, a lens system 6 having a numerical aperture NA of about 0.54 may be used for ultraviolet rays of 436 nm in wavelength .lambda., for example, and in this case, the depth of focus DOF is about 1.5 .mu.m. On the other hand, the thickness of the resist film 8 is also about 1.5 .mu.m, substantially equal to the depth of focus DOF. Therefore, the defectively photosensitized region is inevitably caused in the resist film 8, if the difference in level is formed in the resist film 8.